This invention relates to the thermal stabilization of polyvinylchloride. In particular this invention relates to compositions capable of providing important thermal stability to food grade polyvinylchloride. An important aspect of this invention is a thermal stabilizer composition not containing tin and capable of protecting food grade polyvinylchloride from discoloration when exposed to temperatures of at least about 450o F. during processing into articles such as tamper evident closures.
In recent years a number of patents have issued showing how to stabilize polyvinylchloride resins using organotin compounds. Among these patents are U.S. Pat. No. 3,803,083, issued Apr. 9, 1974 to Brecker; U.S. Pat. No.3,887,519, issued June 3, 1975 to Weisfeld; U.S. Pat. No.4,255,320 issued Mar. 10, 1981 to Brecker; and U.S. Pat. No.4,839,992, issued May 30, 1989 to Larkin. Organotin compounds, because of their unusual heat stabilizing properties, have now set a standard for heat stability which remains unequaled. However, the organotin compounds have the disadvantage that they are odorous, and this limits their use to applications where odor is not a problem.
Solutions to the problem of the heat instability of polyvinylchloride resins which do not use tin-containing compounds have also been disclosed in the literature.
Crochemore and Gay, U.S. Pat. No. 4,102,839, propose compositions for stabilizing vinyl chloride polymers against thermodegradation composed of a divalent metal salt of an aliphatic or organic carboxylic acid, in combination with a dicarbonyl compound such as a beta-diketone or a beta-ketoaldehyde.
Gay U.S. Pat. No. 4,123,399 discloses thermal stabilizer compositions containing a pair of organic acid salts of calcium and zinc, a polyol, and a beta-diketone of like formula to U.S. Pat. No. 4,102,839. The working Examples of the Gay patent also demonstrate the important contribution of the beta-diketone to heat stability during high temperature processing of the polymer chloride resin composition.
Gay U.S. Pat. No. 4,123,400 discloses thermal stabilizer compositions similar to the disclosure in his U.S. Pat. No. 4,123,400 except that the organic acid salts are pairs of the following metals: Ca-Zn, Ca-Cd, Ba-Zn or Ba-Cd.
Minagawa, Sekiguchi and Nakazawa U.S. Pat. No. 4,221,687 provides anti-yellowing additives for environmentally acceptable stabilized vinyl chloride polymer compositions from which arsenic, cadmium, lead, mercury and thallium are substantially excluded, comprising at least one basic inorganic compound of lithium, sodium, potassium, magnesium, calcium, strontium, barium, zinc, titanium, aluminum, zirconium or tin, and a 1,3-diketone compound.
Minagawa, Sekiguchi and Nakazawa U.S. Pat. No. 4,244,848 provides environmentally acceptable stabilizer compositions for enhancing the resistance to deterioration upon heating at 175.degree. C. of a vinyl chloride polymer from which lead, cadmium, mercury, thallium and arsenic are substantially excluded, comprising at least one zinc, alkali metal or alkaline earth metal organic phosphate ester salt and at least one beta-diketone compound having from 5 to about 30 carbon atoms, which is a cyclic or open chain beta-diketone, or a zinc, alkali metal or alkaline earth metal salt thereof.
These stabilizer compositions are used to impart heat resistance to the polyvinylchloride polymer at the high temperatures of at least about 350.degree. F. incurred during the manufacture of various shaped articles by molding, extrusion and plastic processes. These processes require high temperatures in order to bring the polymer to a sufficiently soft state, and the degradation of the polyvinylchloride resin occurs at these processing temperatures.
It is therefore an object of the present invention to provide compositions useful for the stabilization of polyvinylchloride resin from degradation caused by high temperatures.
Another object of the present invention is a food grade polyvinylchloride resin composition not discolored by temperatures incurred during processing.
Another object of the present invention is a useful polyvinylchloride resin thermal stabilizer that does not contain tin.
Still another object of the present invention are methods for preventing the discoloration of polyvinylchloride during its processing into objects used in contact with food.
Other objects will become apparent from the ensuing description.